The debate for the primary mechanism contributed to RCT has been discussed for many years but the answer is still not clear. From an extrinsic point of view, the impingement formed by both acromion and humeral head is a key procedure to increase pressure on tendons. Therefore, the role of humeral head in the progression of the illness should be the same important as that of acromion. A superior humeral translation relative to glenoid was observed in patients with subacromial impingement syndrome in previous kinematic study13. As the superolateral humeral bony projection, the GT is very likely to compromise the subacromial space when abducting or elevating arm. It is very necessary and urgent to discover the relationship between the morphological characteristics of the GT and the formation of RCT for a better understanding of the whole pathological process.
In our study, significant differences were found in the values of GTRR between the two groups, however, no significant differences were observed when comparing the r or the R. This was a dramatic result and had not been discussed in any other researches before. Previous studies indicated that geometric parameters of humeral head, such as the radius of curvature, the articular surface diameter, the articular surface thickness and so on, could be different according to the differences of race, sex, age, height and weight14,15,16. So the values of r or R could differ from person to person as a result of demographic diversity. The Pearson correlation analysis indicated that there was a moderate positive correlation between the values of r and R, which meant the R could change according to the variation of the r. In most cases, a larger value of r usually correlates to a larger value of R. Based on these backgrounds, it is reasonable to realize that a normal person who is higher and heavier could even have a larger humeral head (namely a higher value of r or R) than a patient who is diagnosed with subacromial impingement syndrome but is shorter and thinner. Taking into account the differences of age (though not significant in this study), sexual ratio, height and weight of our cohort, we believed that the results involved with the values of r or R were not sufficient to reflect the truth, and the bias did exist. In order to get a more accurate result, we need a cohort whose demographic characteristics are almost the same, which is hard to achieve in reality.
As a highlight of our research, the design of the GTRR may be a solution for the bias produced by demographic diversity. According to the backgrounds mentioned above, we know that the values of r and R are associated with not only the illness itself, but also the individual differences. With the utilization of division method, we could reduce the influence brought by individual differences and make the results present more information about the illness. The value of R is a direct parameter to measure superolateral extension of the GT, whereas the value of GTRR, which is calculated by division of the values of R and r, is indirect measurement for the extension. For two persons with the same values of r, the one who has larger superolateral extension of the GT will have a higher value of R, resulting in a larger GTRR when comparing with the other. No significant differences found in the R was a conflicting result compared to the significant differences found in the GTRR. This contradiction revealed a fact that the GT indeed contributes to the mechanism of RCT and, at the same time, confirmed the effectiveness of the GTRR on reducing the bias produced by demographic diversity. The indirect measurement was proved to be more effective and practicable than the direct measurement in assessment of superolateral extension of the GT.
In the last decades, many researches about radiographic image have been conducted to reveal the geometric characteristics of shoulders with RCTs. The three subtypes of acromion shapes (flat, curved or hooked) described by Bigliani et al. in 198617, the lateral acromion angle proposed by Banas et al. in 199518, and the critical shoulder angle introduced by Moor et al. in 201319, were all some parameters focusing on the aspect of acromion. In contrast, the researches concentrating on proximal humerus were relatively fewer. The acromion index presented by Nyffeler et al. in 2006 was a parameter involved with lateral border of humeral head20. In 2018, Cunningham et al. designed the greater tuberosity angle (GTA) to evaluate the superolateral extension of the GT, and announced that a GTA value ༞70° was strongly associated with RCT21. For most measurements performed on radiographs, a standard anteroposterior view of shoulder is necessary. In our study, we used 3D models established by reconstruction of CT scans to accomplish measurements. Though adequate positional adjustment of shoulder mentioned above, we could get an ideal standard anteroposterior view and ensure the accuracy and reproducibility of measurements. The Cronbach’s alpha coefficients for both r and R were satisfactory and proved good reliability and repeatability of our methods. Disadvantages of analysis by CT scans compared with X-ray images are high costs and complicated manipulating procedures. Therefore, simple X-ray examination should be preferred in clinical practice, and we aimed to verify our findings by radiographs in further researches.
Although tuberoplasty is not as popular as acromioplasty in treatment of RCT, its satisfactory clinical outcomes have been reported in several studies. Obvious improvement in clinical symptoms and range of motions in patients with massive irreparable RCTs after tuberoplasty combined with subacromial decompression within a two-year follow up was confirmed in some previous work22,23. In another follow up lasting for at least seven years after surgeries with isolated tuberoplasty in patients with massive irreparable RCTs, the researchers also observed good outcomes and regarded tuberoplasty as a good option for relieving pain and improving functionality24. With the discovery of this study, we recommended the GTRR as a postoperative control marker to assess surgical procedures. Decortication and bone removal of the GT should be performed to make the value of GTRR lower than 1.262. However, surgeons must take care not to cause excessive medialization of the GT because it may decrease the deltoid wrapping effect and increase the load of tendons21. More biomechanical researches and clinical observations were needed to reveal the relationship between the GT and the RCTs.
There are some limitations in this study. First, the AUC calculated from the ROC curve of the GTRR was 0.686, and the sensitivity and specificity of the optimized cutoff value (namely 1.262) were 72% and 65%, respectively. These results indicated that the power of the GTRR to discriminate the RCT and control groups was not so excellent. A larger simple size could be helpful to improve the quality of analysis and make the results more accurate. Remembering that the impingement is based on both the GT and acromion, we advise a combined diagnosis by using the GTRR united with an index presenting the geometric characteristics of acromion, such as the critical shoulder angle, to increase the diagnostic sensitivity and specificity. The clinical practicability of the combined diagnosis will be checked in our next work.
The second limitation is that we did not assess the stability of measurements according to different rotation of humerus. In Cunningham et al.’s research about the GTA21, they concluded that the GTA variation remained within a stable range of 1° between − 20° and + 40° of rotation in the axial plane and between − 10° and + 20° of rotation in the sagittal plane. In our study, the humerus was placed in neutral rotation and excellent stability and reproducibility of the values of GTRR were confirmed. However, if a stable measurement range of GTRR according to different rotation of humerus could be defined, it would be helpful to expand the application scope in clinical practice. More efforts are needed to explore the practical effectiveness of the GTRR in clinical diagnosis of RCTs.
The third limitation is that the GTRR was measured in coronal plane and did not take into account the anteroposterior relationship between the GT and the humeral head, whose potential influence may cause bias to the practicability of GTRR.
The last limitation is that we did not perform multivariable analysis because of the lack of data about height and weight of the cohort. Although the significance was not clear, we believed the differences of height and weight existed and contributed to the demographic diversity more or less.